1. Field of the Invention
The present invention relates to an electron beam exposure device and an exposure method using the same in which fine patterns used for an integrated circuit are formed on a wafer utilizing a charged particle beam such as an electron beam.
2. Description of the Related Art
In the recent times, along with the production of highly integrated circuits, the photolithographic system, which had been the main fine pattern forming method for a long time, has been replaced with a new exposure method utilizing an electron beam.
The conventional exposure device utilizing a charged particle such as an electron beam forms a pattern by deflecting and scanning an electron beam having a variable rectangular cross-sectional shape on a sample wafer.
This conventional device generally has a pattern generating function and forms a pattern utilizing pattern data software.
In this device, a pattern is usually formed by assembling a plurality of exposure shots, each of rectangular configuration.
As the pattern size becomes smaller, the number of shots of the electron beam in an unit area increases.
Therefore, a problem arises in that the through-put is reduced.
In order to overcome this problem, a block exposure system has been proposed.
Generally speaking, most parts of an ultra fine pattern used in a semiconductor device such as a 64 MDRAM are formed by arranging certain basic patterns repeatedly and successively.
Therefore, if a basic pattern of a certain complexity of pattern shape, i.e., a "block", can be generated in one operation and an electron beam having a certain cross-section shot on to a wafer, a certain pattern can be exposed on a wafer with a predetermined constant through-put despite the fineness of the patterns.
To obtain the overall pattern, several basic patterns, each having a different cross-section, are provided on a mask.
The intended overall pattern is formed by exposing the basic patterns on the wafer repeatedly and combining them.
FIG. 1 shows a conventional block-exposure system.
In FIG. 1, an electron gun 3a is provided in a charged particle beam generating means 3. An electron beam emitted from the electron gun 3a is given a rectangular cross-section utilizing a plate 6.
The plate 6 has an aperture which gives the cross-section of the electron beam a first rectangular configuration.
The electron beam thus formed is deflected from a center of the beam path by an aperture selecting deflector 5 and irradiated on a selected basic pattern formed on a stencil mask 27.
The electron beam passing through the mask 27 is contracted by a contracting lens 14, and then irradiated on a wafer 25 through a projecting lens 24 and deflecting systems 21 and 23.
FIG. 2 shows one example of a conventional stencil mask 27.
As shown, a pattern forming portion has a thin portion like a thin film.
Several different kinds of basic patterns P.sub.1 to P.sub.3 are formed on the plate 27 utilizing an etching process.
As a mask substrate, a semiconductor such as Si or a metal plate can be used.
In this electron beam exposure device, the electron beam given the same cross-sectional configuration as a pattern on the pattern mask has a certain current value corresponding to the size of the cross-sectional configuration.
The image-forming condition for the pattern image focused on the surface of the wafer slightly changes in response to the current value measured on the surface.
Note that the size of an electron beam passing through any one of the apertures can be set by the patterns P.sub.1 to P.sub.3. An electron beam passing through a pattern with a large aperture area generally has a large current while an electron beam passing through a pattern with a small aperture area has only a small current.
When such an electron beam is irradiated on a wafer 25 mounted on a sample holder 26, the edge sharpness of the beam is sometimes lost due to a difference in the electron scattering effect caused by a difference of the current, between a shot utilizing a large aperture pattern and one utilizing a small aperture pattern.
Accordingly, a problem arises that sharp edges cannot be obtained.
To resolve this, it has been proposed to provide a focus adjusting coil, i.e., a refocus coil 28, between the mask plate 27 and the sample holding device 26.
In this system, however, the problem still remains as to how the focus adjusting value can be determined with respect to each of the block patterns P.sub.1 to P.sub.3.
That is, in the block-exposure device, when a plurality of block patterns having different aperture areas are used, since the beam size changes in accordance with the selected block pattern and thus the current, i.e., the detected electric current, passing through a surface of the sample may changed when the beam is irradiated on the wafer, the conventional system requires a focus adjusting operation utilizing a refocus coil.
In this conventional focus adjusting system, however, while the sharpness of the beam edge can be relatively easily measured with a rectangular beam since the cross-sectional configuration of the beam obtained by the variable rectangular shape generating means is rectangular and thereby the focus adjusting value can be determined in response to the beam size (current flowing through surface of sample), it is very difficult to determine the focus adjusting value since it is not easy to measure the sharpness of a block pattern where the beam cross-sectional configuration is generally complicated.
Another system can be considered in which exposure data representing aperture areas of each of plural block patterns are previously stored in a suitable data storing means, a focus adjusting value is determined from the aperture area and the current density, and the focus adjusting value thus determined is applied to a refocus coil.
In this system, however, another problem arises in that the processing circuit and process control circuit for determining the focus adjusting value, i.e., adjusting data, are very complicated.